A polymer material represented by polyphenylacetylene (PPV) and polyvinyl carbazole (PVK) was applied to solution processed OLED at the earliest, which has a good solubility and excellent film-forming properties, but has disadvantages that the polymer is actually a mixture, with a molecular weight failing to be accurately controlled and poor reproducibility, and is difficult to be purified, thereby greatly affecting the device performance.
Since 2012, small molecules become new favorites for solution processing devices, and in the field of bipolar host, a “phosphorescent” spin coating device is manufacturing by vapor-depositing small molecules, so as to achieve 18.0% external quantum efficiency. But because small molecules have the problems of poor film-forming properties, easily dewetting, and fluid characteristics highly dependent on solvent, developing small molecule materials having excellent solubility, good film-forming properties and being reliable when forming devices is still a great challenge.
The technology of a solution-processable TADF (thermal active delay fluorescent) device has gradually become a hotspot since 2015. The prior art has reported an axisymmetric star-shaped soluble molecule, which has a ΔEST of only 0.02 eV and very typical TADF properties. The external quantum efficiency of spin coating devices with it as the light emitting layer is as high as 5.2%, but it is still less satisfactory in the aspects of electrical properties and film-forming properties, etc.